Cultivated lettuce belongs to the highly polymorphic species, Lactuca sativa, which is a member of the Asteraceae (Compositae) family. L. sativa is one of approximately 300 species in the genus Lactuca. Lettuce is related to chicory, sunflower, aster, acorzonera, dandelion, artichoke and chrysanthemum. Lettuce is commercially grown for its edible head and leaves, wherever environmental conditions permit the production of an economically viable yield.
TSWV and INSV are closely related but distinct members of the genus Tospovirus. Since viruses such as TSWV and INSV are obligate parasites, they are unable to survive outside of their host species. Transmission of these viruses occur from one plant to another by seed, insects or cuttings. Six thrips species are known TSVW vectors: Western flower thrips, onion thrips (Thrips tabaci), tobacco thrips (Frankiniella fusca), common blossom thrips (Frankliniella schultzei), Thrips setosus, and Scirtothrips dorsalis. In the case of INSV, the insect vector is Western flower thrips (Frankliniella occidentalis). Both viruses are highly polyphagous and can cause total crop losses in a number of cultivated vegetable crops including lettuce, tomato, and pepper. In lettuce, TSWV and/or INSV infection has been confirmed on cos or romaine, crisphead or iceberg, and greenleaf lettuces, but essentially these viruses are capable of infecting all types of lettuces.
Symptoms of TSWV and INSV may vary depending on the host, environmental conditions affecting the host, and the individual virus infecting the plant. Common symptoms that may affect all hosts include necrotic spots, streaking, ring spots, stunting, and wilting.
In lettuce, TSWV and/or INSV can cause indistinguishable symptoms. The leaves of infected lettuce plants develop brown spots and necrotic areas. As necrosis spreads, much of the leaf browns, dries out, and dies. Margins of leaves may wilt and become yellow. Leaf browning and yellowing is evident on both newer and older leaves. Often only one side of an infected plant is affected. Lettuce plants that are infected early in development may become stunted and then die. Infected plants that survive to harvest are usually unmarketable. Additionally, TSWV infected lettuce plants produce limited or no seeds. Diagnosis is difficult based on symptoms alone, as these may resemble other fungal and bacterial diseases, damage caused by fertilizer or pesticide applications, or environmental stresses.
Currently, methods to control TSWV and INSV are mainly targeted towards the thrips vectors, or may involve the application of sanitation measures. TSWV and INSV may be controlled with good management practices that reduce the likelihood of thrips infestation, inspecting and isolating plants that present virus symptoms and/or thrips, destroying such symptomatic plants, and monitoring and managing thrips levels. Although chemical control of thrips is possible, repeated use of certain insecticides has led to resistant thrips populations. Therefore, it is highly advantageous to find direct sources of resistance to TSWV and/or INSV, rather than managing the virus vectors or employing control measures once infectivity has already set in.
One approach to introduce TSWV resistance to lettuce is through genetic engineering. Transgenic lettuce plants expressing the nucleocapsid (N) protein gene of the lettuce isolate of TSWV were protected against TSWV isolates. Resistance occurs either through accumulation of high levels of transgenic N protein or by an N transgene silencing mechanism activated by its overexpression. Although transgenic methods for introducing resistance may prove to be effective, under the current environment it may not be a widely accepted method, especially when the end product will be used for food consumption.
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